Method of preparing mixed diesters



Unit

3,333,405 METHOD OF PREPARING MIXED DHESTERS Paul J. Sniegoski, Bethesda, Md assignor to the United States of America as represented by the Secretary of the Navy No Drawing. Filed Aug. 31, 1965, Ser. No. 484,131 4 Claims. (Cl. 269-485) where R is an alkylene radial having from 3 to 11 carbon atoms, R is a branched chain alkyl radical having from 5 to 10 carbon atoms, R is a straight chain alkylene radical having from 1 to 3 carbon atoms and R is hydrogen or chlorine.

The mixed diesters are described and claimed in application, Ser. No. 477,984, of Curtis R, Singleterry, filed August 6, 1965. They are oils of low volatility, low pour point and low viscosity-temperature coefficient and nonspreading. They have a surface tension above 32 dynes per centimeter and a maximum viscosity in the range of from about 16,000 to 25,000 cs. at -65 F. and are useful in the lubrication of line mechanisms such as altimeters, chronometers and gyro compasses which encounter deep cold in service aboard aircraft in high altitude flight or satellites in orbit.

The acid portion of the mixed diesters may be derived from the aliphatic dicarboxylic acids of from 5 to 13 carbon atoms which are glutaric, adipic, pimelic, subaric, azelaic, sebacic, hedecanedioc, dodecanedioc and brassylic acids. The one alcohol portion of the mixed diesters may be derived from the corresponding primary aliphatic alcohol, e.g., from 2,2-dimethyl propanol, 2-ethyl butanol, Z-methyl hexanol, Z-ethyl hexanol, 2-methyl octanol, 2- methyl nonanol, etc. The other alcohol portion of the mixed diesters may be derived from the corresponding primary phenylor monochlorophenyl alkanol in which the alkanol portion has a straight chain hydrocarbon group of 1 to 3 carbon atoms, e.g., from benzyl alcohol, 2- phenylethyl alcohol, 3-phenylpropyl alcohol, 2-, 3- and 4-chloro'benzyl alcohol, etc. in a preferred group of the mixed diesters, the acid residue is that of azelaic acid and the single branched chain allryl residue that of 2-ethylhexanol-l. A preferred mixed diester is Z-ethyl'nexyl benzyl azelate. The preferred group of mixed diesters are prepared from the half ester, Z-ethyl hexyl hydrogen azelate.

The mixed esters may be prepared in known way by subjecting a reactant mixture containing the corresponding branched chain alkyl half ester of the alkane dicarboxylic acid of the general formula:

R'OOC-RCOOH wherein R is R are as set forth above, and the phenyl alkanol or monochlorophcnyl aikanol of the general formula:

@RHOII States Patent 0 wherein R and R are as set forth above, in equimolar proportions under esterifying conditions, using a small amount of an acid esterification catalyst such as p-toluene sulfonic acid monohydrate and concentrated sulfuric or phosphoric acid and a solvent-diluent, such as benzene or toluene, for azeotropic removal of water of the reaction. However, the esterification reaction with a stoichiometric mixture of the half ester and the phenylor chlorophenyl alkanol will produce in addition to the mixed diester considerable amounts, generally about 10 mole percent of each of two symmetrical diesters which are the his (branched chain alkyl) diester and the his (phenylalkyl) diester or his (monochlorophenylalkyl) diester, and thus entail a diffcult separation to recover the mixed diester in a pure condition. Separation of the mixed diester from the three-component product mixture by distillation is difficult and inefficient since the boiling points of all three compounds are within a relatively narrow spread at 0.1 mm. Such a separation is further complicated by the possibility of ester-ester interchange, or disproportionation of the mixed diester to the unwanted symmetrical diesters at the temperature of distillation.

The present invention has as an object the provision of an improved method for the preparation of the aforesaid mixed diesters whereby they may be obtained in a pure condition.

The above and other objects are accomplished by the practice of the method of my invention which is hereinafter described.

I have found in the course of my investigations of the aforesaid known synthesis for the preparation of the mixed diesters that the symmetrical diester which is either the his (phenylalkyl) diester or the his (monochlorophenylalkyl) diester may be readily separated from the three-component product mixture in a simple, direct manner and that the aforesaid known synthesis can be operated under conditions to minimize the production of the other symmetrical diester, the bis(branched chain alkyl) diester, whereby the mixed diester is obtained in a pure condition in which the amount of the his (branched chain alkyl) diester is about 1.5 mole percent and of the his (phenylalkyl) diester or bis (monochlorophenylalkyl) diester is about 4 mole percent.

In the practice of the method of my invention, the csterification reaction between the branched chain alkyl half ester and the phenyl alkanol or monochlorophenylalkanol is conducted as described above but with a starting reaction mixture which contains a large excess of the alkanol which is five and may be ten or more times the theoretical or stoichiometric amount for the esterification reaction. The effect of the large excess of the alkanol in the reaction mixture is to substantially increase the rate of formation of the mixed diester and substantially de crease the rate of formation of the symmetrical branched chain alkyl diester. Also, since the esterification of the half ester with the alkanol is carried out almost to completion in a much shorter time than with only stoichiometric amounts of the reactants, the unwanted side reactions which produce the his (branched chain alkyl) diester are allowed a much shorter time to run. A preferred amount of the alkanol for the starting reaction mixture is five times the stoichiometric amount for the reaction, To minimize alcoholysis of the formed mixed diester, the reaction is terminated when substantially all, approximately 98 percent, but not all of the theoretical amount of water of the esterification is collected.

The three-component product mixture from the esterification reaction is neutralized following known practice by washing with dilute aqueous alkali, for example, with 0.5% aqueous potassium hydroxide solution, whereby acid constituents are neutralized and removed from the 3 product mixture. The solvent-diluent is flash evaporated from the product mixture and the unreacted alkanol removed therefrom by molecular distillation.

The washed and stripped three-component product mixture is subjected to a separation step for removal of the symmetrical aromatic alkyl diester therefrom. This separation is accomplished by dissolving the product mixture in a large volume of an alkane which has a freezing or crystallizing point which is substantially below that of the diester to be separated, for example, a C to O; nalkane, and deeply cooling the solution, for example, to -65 to 80 C., to crystallize out the diester which is separated by filtration. The filtrate may be recooled and again filtered for removal of further amounts of the symmetrical diester. The alkane may be n-pentane, n-hexane or n-heptane and is used in a volume which may be from about to 20 times the volume of the product mixture. The alkane is removed by evaporation and the residue distilled on the centrifugal molecular still at low pressure to obtain the mixed diester in a pure condition as described above.

The branched chain alkyl half esters of the alkane dicarboxylic acids which are used as the starting materials for the mixed diester synthesis of the invention may be prepared by the procedure described by James Murphy and ORear, Ind. Eng. Chem. 51:673 (1959).

The method of the invention is further illustrated by the preparation of Z-ethylhexyl benzyl azelate which is a preferred member of the class of mixed diesters. The half ester, 2-ethyl hexyl hydrogen azelate, used in the synthesis was prepared by the procedure of James, Murphy and ORear, above, from azelaic acid and 2-ethyl hexanol-l.

EXAMPLE A mixture of 731.3 grams (2.34 moles) of the 2-ethylhexyl hydrogen azelate, 1315 grams (12.17 moles) of benzyl alcohol, 2 grams of p-toluene sulfonic acid monohydrate and 4 pints of toluene is refluxed for 2 hours until 43 ml. of water is collected in a water trap.

After the product mixture from the esterification reaction is neutralized by washing several times with 0.5% aqueous potassium hydroxide solution in a separatory flask, the toluene is removed by flash evaporation. The unreacted benzyl alcohol is removed by molecular distillation at 60 C. and 16 microns.

A solution of the residue in 5 liters of n-heptane (about 10:1 volume ratio) is cooled to 80 C. and the solid, crystallized dibenzyl azelate removed by filteration. The filtrate is recooled and any further solidified dibenzyl azelate removed by filtration. The heptane is evaporated and the residue distilled on the centrifugal molecular still at 85 to 100 C. and 10 microns. Yield about 587.5 grams of Z-ethylhexyl benzyl azelate which contained about 1.5 mole percent of his (2-ethylhexyl) azelate and about 4 mole percent of his benzyl azelate.

The method of the present invention is general in its application to the preparation of the aforesaid mixed diesters in a pure condition as described above. Thus, for example, 2-ethylhexyl phenylethyl azelate, 2-ethylhexyl Zahlorobenzyl azelate, 2-ethylhexy1 benzyl sebacate, 2- methylhexyl benzyl sebacate, 2-methyloctyl benzyl subarate, etc., may be prepared in a pure condition as described above following the procedure of the above example from the corresponding branched chain alkyl half ester and the corresponding phenylor monochlorophenyl alkanol.

While in the above description of the invention reference has been made to certain specific embodiments thereof, the same is intended by way of illustration and not in limitation except as may be required by the appended claims.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. A method of preparing mixed diesters of the general formula:

wherein R is a straight chain alkylene radical having from 3 to 11 carbon atoms, R is a branched chain alkyl radical having from 5 to 10 carbon atoms, R" is a straight chain alkylene radical having from 1 to 3 carbon atoms and R is a member of the group consisting of hydrogen and chlorine by the esterifioation of a branched. chain alkyl half ester of the general formula:

wherein R and R are as above with an alkanol of the general formula:

wherein R" and R are as above, which comprises heating a reactant mixture of said branched chain alkyl half ester and a large excess of said alkanol which is from about 5 to 10 times the stoiohiometric 'amount for the esterification reaction in the presence of a small amount of an acid esterification catalyst and of a water-immiscible organic solvent for the reactants which forms an azeotrope with water, said water immiscible organic s0lvent being present in amount suflicient to carry off all the water of the reaction, and azeotropically distilling otf the water of reaction until substantially all, but not all of the same is removed from the reaction mixture, said esterificati'on react-ion producing in addition to the mixed diester, a small amount of the corresponding symmetrical branched chain alkyl diester and relative to the latter, a larger amount of the corresponding aromatic alkyl symmetrical diester, neutralizing the reaction mixture from the esterificat-ion with dilute aqueous alkali, evaporating the volatile organic solvent from the neutralized reaction mixture and distilling the unreacted alkanol therefrom, dissolving the residue in a large volume of an n-alkane having from 5 to 7 carbon atoms, cooling said alkane solution to a temperature substantially below that at which the formed symmetrical aromatic alkyl diester crystallizes, filtering the crystallized symmetrical diester from solution and distilling the filtrate under reduced pressure to recover the mixed diester.

2. A method as defined in claim 1, wherein the half ester is a branched chain alkyl half ester of azelaic acid.

3. A method as defined in claim 2, wherein the half ester is Z-ethylhexyl hydrogen azelate.

4. A method as defined in claim 1, wherein the acid esterification catalyst is p-toluene sulfonic acid monohydrate.

References Cited UNITED. STATES PATENTS 3,172,904 3/1965 Rehfuss 260-485 LORRAINE A. WEINBERGER, Primary Examiner.

T. L. GALLOWAY, Assistant Examiner. 

1. A METHOD OF PREPARING MIXED DIESTERS OF THE GENERAL FORMULA: (R"''-PHENYL)-R"-OOC-R-COO-R'' WHEREIN R IS A STRAIGHT CHAIN ALKYLENE RADICAL HAVING FROM 3 TO 11 CARBON ATOMS, R'' IS A BRANCHED CHAIN ALKYL RADICAL HAVING FROM 5 TO 10 CARBON ATOMS, R" IS A STRAIGHT CHAIN ALKYLENE RADICAL HAVING FROM 1 TO 3 CARBON ATOMS AND R"'' IS A MEMBER OF THE GROUP CONSISTING OF HYDROGEN AND CHLORINE BY THE ESTERIFICATION OF A BRANCHED CHAIN ALKYL HALF ESTER OF THE GENERAL FORMULA: 